Antenna Aperture - Effective Length

Effective Length

For antennas which are not defined by a physical area, such as monopoles and dipoles consisting of thin rod conductors, the aperture bears no obvious relation to the size or area of the antenna. An alternate measure of antenna gain that has a greater relationship to the physical structure of such antennas is effective length l_eff measured in metres, which is defined for a receiving antenna as:

where

V₀ is the open circuit voltage appearing across the antenna's terminals

E_s is the electric field strength of the radio signal, in volts per metre, at the antenna.

The longer the effective length the more voltage and therefore the more power the antenna will receive. Note, however, that an antenna's gain or A_eff increases according to the square of l_eff, and that this proportionality also involves the antenna's radiation resistance. Therefore this measure is of more theoretical than practical value and is not, by itself, a useful figure of merit relating to an antenna's directivity.